The present invention relates to a fixed catalyst bed suitable to be used in a Fischer-Tropsch process, in particular to a fixed bed which is able to withstand a process for carrying out a high-speed stop in a Fischer-Tropsch process. The present invention further relates to the use of the fixed bed, and to a Fischer-Tropsch process in which the fixed bed is used.
The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed-stocks into normally liquid and/or solid hydrocarbons (0° C., 1 bar). The feed stock (e.g. natural gas, associated gas, coal-bed methane, residual oil fractions, biomass and/or coal) is converted in a first step into a mixture of hydrogen and carbon monoxide. This mixture is often referred to as synthesis gas or syngas. The synthesis gas is fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight hydrocarbons comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi-tubular fixed bed reactors, fluidised bed reactors, such as entrained fluidised bed reactors and fixed fluidised bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebullated bed reactors. WO2008089376 discloses a Fischer-Tropsch microchannel reactor comprising a plurality of Fischer-Tropsch process microchannels and a plurality of heat exchange channels. A microchannel is defined in WO2008089376 as a channel having at least one internal dimension of height or width of up to about 10 mm. The Fischer-Tropsch catalyst in the microchannels may be a graded catalyst. The graded catalyst may have a varying concentration or surface area of a catalytically active metal. The graded catalyst may have physical properties and/or a form that varies as a function of distance.
The Fischer-Tropsch reaction is very exothermic and temperature sensitive. In consequence, careful temperature control is required to maintain optimum operation conditions and desired hydrocarbon product selectivity.
The fact that the reaction is very exothermic also has the consequence that when temperature control is not adequate, the reactor temperature can increase very quickly, which carries the risk of a reactor runaway. A reactor runaway may result in highly increased temperatures at one or more locations in the reactor. A reactor runaway is a most undesirable phenomenon, as it may result in catalyst deactivation which necessitates untimely replacement of the catalyst, causing reactor downtime and additional catalyst cost.
A high-speed stop may, for example, be required when the temperature in the Fischer-Tropsch reactor increases to an unacceptable value either locally or over the entire reactor, when there is an interruption in the gas flow, or in the case of other unforeseen circumstances. When there is a threat of a runaway, it is often wise to stop the reaction as quick as possible. Several processes for carrying out a high-speed stop in a Fischer-Tropsch reactor have been developed.
The desired use of highly active and less diffusion limited catalysts in Fischer-Tropsch fixed-bed reactors makes the situation even more challenging. The susceptibility to a runaway increases with increased catalyst activity and with reduced diffusion limitation of the catalyst. Examples of methods that are especially suitable for Fischer-Tropsch fixed-bed reactors comprising highly active and less diffusion limited catalysts can be found in WO2010063850, WO2010069925, and WO2010069927.
When a high-speed stop is carried out in a fixed-bed reactor, a raise in temperature, culminating in a process-side temperature peak is often observed. If a process-side temperature peak is observed, it is usually observed at the upstream side of the catalyst bed.
A process-side temperature peak is generally caused by a decrease in gas space velocity which leads to an increased conversion, accompanied by increased heat formation, and simultaneously to a decrease in heat removal capacity.
The peak temperature increase can be minimized by choosing the right method for the high-speed stop, but it will nevertheless have some influence on the catalyst bed. Especially when less diffusion limited catalysts in Fischer-Tropsch fixed-bed reactors are applied, the conditions during a high-speed stop are critical.
Therefore, there is need for a Fischer-Tropsch fixed-bed which is better able to withstand any kind of process for carrying out a high-speed stop in a Fischer-Tropsch process.